DOWNEY JR steels, sorry, steals the show in this latest cinematic plundering of the Marvel comics universe. He serves up swagger and swank and arch one-liners in place of the earnestness of Spider-Man, or DC's dour Batman. His charismatic performance holds Jon Favreau's film together when it threatens to lose its way between the crash-bang set pieces. Initially, Downey Jr is Tony Stark, a Scotch-swigging, philandering, billionaire arms manufacturer. Wounded by his own ordnance and captured in what is clearly Afghanistan - though Favreau keeps the politics deliberately vague - Stark escapes by building a suit of robotic armour around a generator implanted in his chest to keep his injured heart going. Back home he starts to question his trade, and builds a new suit with which to destroy his mis-sold armaments. He's hard on the outside, soft inside, and we know his heart's in the right place because we can see it glowing, like ET's, see? Fortunately, Downey Jr's Stark doesn't stop being flippant or flash. There are delightfully witty scenes of him trying out his armour like a new sports car, observed only by two dumb robots and a sarcastic electronic butler. Thank goodness for a hero whose flaws are arrogance and cockiness, because elsewhere, the human element is lacking. When Gwyneth Paltrow and Jeff Bridges appear as Stark's assistant-cum-love interest and his mentor-cum-nemesis, the film slows noticeably. While Bridges just happily chews cigars and the scenery, the growing intimacy between Stark and Paltrow's fetishistically-attired but prim "Pepper" Potts feels agonisingly forced. Perhaps Favreau, who wrote Swingers and directed Elf, is happier with jokes and the ironic observation of boys and their toys. Sure enough, we're soon back to Stark's Iron Man, taking a mobile phone call in his helmet while playing aerial tag with a pair of F11 fighter planes. There's a satisfyingly climactic CGI battle, but Downey Jr tops it in the witty denouement with pure acting skill. Favreau's film inevitably recalls other superhero movies because all their storylines follow a broadly similar arc. A more telling problem is that it seems to treat itself as the first instalment in an aticipated franchise. Stark's struggles here are basically internal: only at the very end does he contemplate what it means to be Iron Man. Other characters are sketched in and cued up for future glory. "Next time, baby," says Stark's bland chum Jim (Terrence Howard) as he eyes a spare metal suit. I hope there is a sequel, though. Despite its flaws, the film delivers splendidly tense action sequences, a magnetic performance from Downey Jr, and Paltrow in tight dresses and high heels. Which, for superhero fans, is a near-perfect hat-trick. And now, forgive me if I mourn the success of Martin McDonagh's first feature film, In Bruges. For one thing, I know him very slightly, and whenever a (younger) acquaintance does well, a part of me dies. But mostly, it's because I fear the theatre has lost one of its most bracing, caustic and blackly comic talents to a world where he may feel more at home but shine less brightly. It is an oft-repeated truth that McDonagh barely went to the theatre when he began writing plays in the 1990s. All his references and inspiration came from film. But his first staged script, The Beauty Queen of Leenane, detonated like a bomb when the Royal Court put it on in 1996. It was funny, cruel, merciless in its guying of the peat-bog fictions of JM Synge and John B Keane, and beautifully written. As more funny, violent, sweary plays poured out, some lazily praised or dismissed him as theatre's Tarantino — a label he shrugged off with the politically bold, paramilitary-baiting Lieutenant of Inishmore and the extraordinary dark fantasy about creativity, cruelty and family, The Pillowman. Even those upset by his work admitted he invigorated theatre. He also became one of the few people to make serious money from it. When Six Shooter, a brutally funny short film about dying which he wrote and directed, won an Oscar in 2006, though, I knew the writing was on the wall. And with In Bruges, McDonagh has proved himself a masterly film director This tale of two trapped hitmen — close in tone, ironically, to Harold Pinter's play The Dumb Waiter — shows a deft understanding of pace and pathos as well as mordant wit. McDonagh draws career-best performances from both Colin Farrell and Ralph Fiennes, as well as prodding at taboos by including a racist, ketamine-addicted dwarf who gets his head blown off But hitmen and drugs and arterial spray are more common in the movies than in the theatre. What might seem radical on stage looks like a genre piece on film. Although McDonagh looks set for a brilliant cinematic career, I don't know whether he'll produce anything with the impact of Beauty Queen or the dazzling texture of Pillowman. Since the rewards, the audiences and the potential for creative control are greater in film, I suspect he may not come back. But for his and theatregoers' sake, I hope he does.